1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a membrane electrode assembly and metal separators of corrugated plates in a stacking direction. The membrane electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes. A reactant gas flow field is formed in the fuel cell for supplying a reactant gas along an electrode surface in a direction of gravity or in a horizontal direction. A coolant flow field is formed in the fuel cell for supplying a coolant in a direction intersecting the flow direction of the reactant gas of the reactant gas flow field.
2. Description of the Related Art
For example, a solid polymer electrolyte fuel cell employs a membrane electrode assembly which includes an anode, a cathode, and an electrolyte membrane interposed between the anode and the cathode. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly is sandwiched between separators to form a power generation cell. In use, a predetermined number of power generation cells are stacked together to form a fuel cell stack. p In the fuel cell, a fuel gas flow field (hereinafter also referred to as the “reactant gas flow field”) for supplying a fuel gas along an anode and an oxygen-containing gas flow field (hereinafter also referred to as the “reactant gas flow field”) for supplying an oxygen-containing gas along a cathode are formed in surfaces of separators facing the anode and the cathode, respectively. Further, a coolant flow field for supplying a coolant is formed along surfaces of the separators for each of the power generation cells, or for every predetermined number of the power generation cells.
The fuel cell may adopt so called the internal manifold structure in which reactant gas passages and coolant passages extending in the stacking direction of the separators are provided in the fuel cell. In the structure, in general, buffers are provided between the reactant gas passages and the reactant gas flow field for supplying, and dispersing the reactant gas to the reactant gas flow field uniformly.
For example, in Japanese Laid-Open Patent Publication No. 2002-530836 (PCT Application), as shown in FIG. 8, an oxygen-containing gas inlet manifold 2a, a coolant inlet manifold 3a, a fuel gas inlet manifold 4a extend through a sheet metal element 1 at one end in a longitudinal direction. An oxygen-containing gas outlet manifold 2b, a coolant outlet manifold 3b, and a fuel gas outlet manifold 4b extend through the sheet metal element 1 at the other end in the longitudinal direction.
A straight corrugated flow field 5 is formed in a cooling surface of the sheet metal element 1. An inlet buffer 6a and an outlet buffer 6b each including dimples or rails are provided at opposite ends of the corrugated flow field 5. Though not shown, the inlet buffer 6a and the outlet buffer 6b are provided at positions corresponding to inlet buffers and outlet buffers of flow fields for the fuel gas and the oxygen-containing gas. It is because the flow direction of the coolant is the same as the flow directions of the fuel gas and the oxygen-containing gas.
In the conventional technique, the inlet buffer 6a and the outlet buffer 6b are provided on the cooling surface of the sheet metal element 1. The coolant supplied from the coolant inlet manifold 3a flows from the inlet buffer 6a to the corrugated flow field 5, and the coolant is discharged from the corrugated flow field 5 to the coolant outlet manifold 3b through the outlet buffer 6b. 
However, in the structure, the coolant flows into the inlet buffer 6a and the outlet buffer 6b where power generation is not performed and cooling is not required. Therefore, the flow rate of the supplied coolant needs to be larger than the flow rate of the coolant which is actually required for cooling the power generation area. Thus, loss of electrical energy in the coolant pump is large, and the system efficiency is low.